Amides are important class of organic compounds used in the manufacture of drugs, engineering plastics, detergents and lubricants. (Meth)acrylamide and caprolactum are two amide group-containing monomeric compounds of great industrial relevance in the preparation of polymers. Compounds of amides are known to have excellent anthropod-controlling activity and application in the treatment of HIV disease.
Although there have been several methods to prepare amides, their preparation under neutral conditions without generating waste by-products is a challenging task. Amides are mostly prepared by the reaction of amines with activated acid derivatives (acid chlorides and anhydrides) (Chemical Abstracts, Vol. 75, 1971, abstract no. 129306g). This reaction generates equimolar quantity of acid by-product which needs further processing steps to neutralize and separate from the desired amide product. Further, this reaction becomes sluggish and often fails to take place if the amine is deactivated due to presence of electron, withdrawing substituents in it.
Amide compounds have also been produced at an industrial scale by hydrating the corresponding nitrile at high temperatures over a reduced metal catalyst—Raney Ni and Cu, for example. In recent times, nitrile hydratase-containing microorganisms are also being used in their production (U.S. Pat. No. 6,043,061; EP 1,266,962 A2; EP 1,835,033A1).
There have been some reports on the direct synthesis of amides from alcohols and amines in the presence of metal catalysts (Gunanathan et al., Science, Year 2007, Vol. 317, pp. 790 792; S. C. Ghosh and S. H. Hong, Eur. J. Org. Chem. Year 2010, pp. 4266-4270). Primary amines are directly acylated by equimolar amounts of alcohols to produce amides and molecular hydrogen (the only product) in high yields and high turnover numbers. This reaction is catalyzed by a homogeneous catalyst, ruthenium complex based on a dearomatized PNN-type ligand [where PNN is 2-(di-tert-butylphosphinomethyl)-6-(diethylaminomethyl)pyridine]. No base or acid promoters are required. However, there is a requirement of additional reagent alcohol to produce amides.
Oxygenation of amines is an efficient route for amides synthesis. This transformation possibly proceeds by a tandom process of oxidative dehydrogenation of amines to nitriles, followed by hydration to produce corresponding amides. Kim et al (Angew. Chem. Int. Ed., Year 2008, Vol. 47, pp. 9249-9251) reported the application of alumina-supported ruthenium hydroxide for this transformation. Ruthenium, a precious metal, is a less abundant and expensive metal and hence, is not desirable for use. Wang et al. (Chem. Commun., Year 2012, DOI: 10.1039/c2cc17499e) demonstrated the use of manganese oxide octahedral molecular sieves (OMS-2) catalysts for this reaction. Low hydrothermal stability and durability are the issues with this catalyst. Further, OMS-2 is a microporous catalyst with pore size of 4 to 5 Å. Bulkier amines are, therefore, not amenable for transformation to amides over these prior-art catalysts. Water, a by-product generated during the amide formation reaction deactivates and destabilizes the catalyst. Although, 87% yield of benzylaminde is obtained over fresh catalyst in its first, on reuse the yield of amide dropped down to 82% which is a clear indication of less stability of OMS catalysts during long term usage.
In view of importance of amide compounds in industrial applications and drawbacks of prior-art processes which include use of expensive, low abundant metals, mineral acids or bases for rearrangements, low structural stability and microporosity of catalysts, etc., it is desirable to have a more efficient catalyst process.
Metal-containing porous solid catalysts, especially those of mesoporous silicas, silicates, aluminophosphates and silico-aluminophosphates have been known for their catalytic activity in other organic transformations. These catalysts are known for their high thermal, hydrothermal and mechanical stability during their thick porewalls (20-40 Å). The inventors disclose herein a novel invention wherein ordered, mesoporous non-precious metal-containing catalysts are used for the preparation of amides from amines.